Evaporative ponds dimensioning relies mainly among parameters to local water precipitations from rain, ambient air temperature during the year, and the wind velocity conditions forcing heat convection.
One way to accelerate the evaporation is to increase the temperature of the liquor relatively to the ambient air temperature and its associated dew point. Another way to accelerate the evaporation is to increase the contact surface between air and the liquor comprising water to be partially evaporated. Among those ways, spraying systems enable to divide a liquor into droplets having more evaporation specific surface area than the more or less flat surface of the corresponding liquor stored in a pond. Though operating spraying devices with aqueous solutions comprising water and sodium carbonate and optionally other soluble salts such as sodium bicarbonate, sodium chloride, or sodium sulfate leads to fast scaling and clogging of those spraying devices at the outlet of the spraying devices when water is evaporated.
Among the industries using evaporative ponds, soda ash production from natural ores or from alkaline lakes is one of them. The main natural ores comprising sodium carbonate are: trona, nahcolite or wegscheiderite ores. Those natural ores can be found in different geographical areas such as Brazil, China, Kazakhstan, Mexico, Turkey, USA (Wyoming, Colorado, California). Alkaline lakes are found mainly in Africa. Those natural ores can be mechanically mined, for instance using longwall equipments, or be mined by solution mining with water.
The production of soda ash and its derivatives from natural ore or from alkaline lakes, results in the production of purge streams containing sodium carbonate. Indeed most of the natural deposits contain impurities such as insoluble particles (sand, clay, calcium carbonate, feldspar, . . . ) and soluble salts mainly evaporite salts (sodium chloride, sodium sulfate, . . . ).
The impurities must be removed from the process lines to guarantee a final product quality required for main uses of soda ash: such as glass production, or food and pharmaceutical production of refined sodium bicarbonate. Most of the soluble impurities such as soluble salts or less soluble compounds are removed from the production process as aqueous solutions or aqueous suspension comprising aqueous solution in different purge streams containing sodium carbonate.
Generally the soluble impurities are removed by a purge control during the crystallization steps in order to guarantee the final soda ash or soda ash derivative quality.
Moreover when using a solution mining an higher amount of water is used per ton of soda ash or soda ash derivatives compared to a conventional dry mining operation, increasing therefore the needs to control the water balance and the evaporation energy optimization when combining evaporative ponds and industrial evaporators fed with steam from a steam generator.
U.S. Pat. No. 1,853,275 (1932) discloses a manufacturing process of sodium carbonate from salt residues of alkaline waters in Owens Lake (California). The document teaches to avoid the building and operating of evaporation ponds to store summer brine of the lake. It teaches to use steam to melt crystals of sodium carbonate decahydrate and sodium chloride in a sodium sulfate rich brine and reach the three solid phases sodium carbonate monohydrate, Na2CO3.2Na2SO4 double salt, and sodium chloride to precipitate double salt to decrease sodium sulfate concentration of the brine. The double salt is removed from the resulting brine and purified sodium carbonate decahydrate is then crystallized. The document is silent on the use of indirect heating by using a heat exchanger to enhance the evaporation of the evaporation pond, silent on the use of spraying device and on spraying conditions.
US2003/0143149 discloses a process for recovering the sodium carbonate from evaporative pond water using heat to heat a sodium carbonate recovery stream before introducing it to an evaporation pond (20). The pond liquor is heated at 120-140° F. (49-60° C.) before being causticized with caustic or quicklime. The described process is silent on the use of spraying device and spraying conditions. It is also silent on multi-ponds operation.
The present invention aims to propose an improved method for increasing the evaporation rate of an evaporative pond containing a sodium carbonate solution using spraying device and low temperature heat or waste heat, in particular from a steam generator or from a soda ash plant.